Liquid crystal display devices such as a liquid crystal television are provided with a surface light source device which illuminates a liquid crystal panel from the back side (for example, Patent Document 1). The surface light source device is categorized into two major types: a direct type, in which a light source is disposed in a manner facing an optical member; and an edge light type, in which a light source is disposed on the side of an optical member. The edge-light type surface light source device has an advantage that it can be made thinner, compared with the direct-type surface light source device.
The edge-light type surface light source device is provided with a light guide plate, which faces a liquid crystal panel and guides the source light emitted from the side. The light from the light source enters the light guide plate from the side face (light input face) of the light guide plate. The light that has entered the light guide plate is reflected repeatedly in the light guide plate, and moves through the light guide plate toward the face opposing to the light input face (in a light guiding direction). As the light moves through the light guide plate, it is gradually emitted from the light output face by the optical function of the light guide plate. Accordingly, the amount of light emitted from the light output face of the light guide plate is uniformed along the light guiding direction.